Conventionally, as an apparatus for performing printing on a recording paper such as thermal paper, various thermal printheads have been proposed (See Patent Document 1 below for example). FIG. 11 of the present application shows an example of thermal printhead as related art of the present invention. Specifically, the illustrated thermal printhead B includes an insulating substrate 91, on which a glaze layer 92 made of glass, a heating resistor 93, an electrode 94 and a protective film 96 are laminated. The protective film 96 is made of a material mainly composed of glass. In printing using the thermal printhead B, recording paper such as thermal paper is moved relative to the printhead while being pressed against the protective film 96. In this process, heat generated at the heating resistor 93 is transferred to the recording paper, whereby appropriate printing is performed.
In the above-described thermal printhead, the electrode 94 can be made of a metal having excellent conductivity such as Al, Cu or Au. Of these metals, Au is a chemically stable material and has excellent corrosion resistance. Therefore, when the electrode 94 is made of Au, conduction failure due to the corrosion of the electrode can be avoided. Further, the electric resistance (resistivity) of Au is lower than that of Al and so on. Therefore, when the electrode 94 is made of Au, the amount of voltage drop is smaller than when Al is used, so that the power loss can be made smaller.
Although the electrode made of Au has the above-described advantages, it also has the following drawbacks. As compared with other highly conductive metals like Al, the adhesion of Au to glass which forms the protective film is poor. Therefore, the protective film may separate from the electrode 94, which leads to reduction of durability of the thermal printhead. Further, the difference in thermal expansion coefficient between the electrode and the protective film causes stress to be applied to the protective film, which promotes the separation of the protective film.
Patent Document 1: JP-A-2002-67367